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Soil amending

by editor

Exciting times indeed.
We’ve arrived at a time where governing bodies, science and industry all agree that working with the existing amended site soil will deliver a more durable, water efficient and sustainable landscape asset.

As I mentioned in my maiden article there are some exceptions to this practice such as contaminated sites, roof, or balcony gardens, engineered landscapes like high density residencies and other installations that are totally reliant on human inputs to ensure survival.

Although the practice of top dressing an unprepared site with a landscape soil blend prior to planting still occurs, most of us understand or have realised that the practice and performance of the garden is short-termed and unsustainable.

We’ll cover the reasons for this in future articles about landscape blends. In the meanwhile, let’s bring it back to the topic of site soils.

I personally love the challenge of creating a healthy growing media with the existing soil on hand. Although plants might not grow as fast as they would if they were in a lightweight, sand-based landscape blend. When the site soil is prepared properly to the needs of that species, they are more resilient to adverse conditions, sturdier and much longer lived.

Our professional duty is to create a landscape asset that exceeds the lifespan of the surrounding structures. With this in mind, we need to implement the correct systems and provide the appropriate resources to ensure plant longevity with useful life expectancies.

It’s possibly been a long time since you’ve acquainted this topic or perhaps it’s new to you. Either way, it never hurts to revisit topics relating to soil. One of the two most precious resources we have on Earth. The other being unpolluted water.

Keep in mind that we’re talking about soil, not dirt.

There are five main components that when combined, make a soil.

  1. Mineral particles: elements that were never alive (rock and mineral fragments)
  2. Organic matter: the remains of once living organisms or the refuse from living organisms
  3. Living organisms: These critters range in size from tiny microbes, fungi, earthworms and right up to ground dwelling mammals like wombats, rabbits, rats etc
  4. Water: The element of life in which nutrients for plants are dissolved to become available for absorption
  5. Air: Fills the spaces and voids between the solid particles that are not filled with water

The variation in proportions of the five components, create the differences we see, feel and smell in soils. Plants aren’t that demanding from soil. All they want is their particular balance of water, air and nutritional elements. When air, water and nutrient aren’t in balance for a particular species then it may suffer drought from too much air in the soil or waterlogging from too much water in the soil. It may also suffer drought and suffocation from no water and no air. This situation is usually due to layering or compaction but let’s leave these two avoidable nightmares for next year sometime.

Getting back to soil. There are generally four different types of soil with varying mineral particles and sizes that are the foundations of the soils we work with. Sand, silt, clay and the most desirable, loam. The quantities or proportions of these types is known as texture.

Sand
Sand consists of small particles of weathered rock. Sandy soils are one of the poorest types of soil for growing plants because of its very poor ability to hold nutrients and water, this makes it hard for the plant’s roots to absorb water. Its gritty texture and tendency to fall apart in your hand makes it quite easy to identify. Although it’s useful for growing turf and many coastal native plants, it continually needs water and nutrients to sustain life.

Silt
Silt, compared to sand, has smaller particles which is made of rock and other mineral particles, these are smaller than sand. Silts are better at holding water, nutrients and are far more stable than sandy soil. Turf is grown on silty soil because of its stable nature and ability to hold water. Although it’s useful for the turf farm, the silt is often detrimental to the turf in the landscape. More on this in future articles.

Clay
Clay has the smallest particles of these three. The particles in this soil type are so tightly packed together there is very little air space. It’s heavy in nature and as most of us know, doesn’t drain well. It does, however, have an ability to hold nutrients and water. All good landscape soils should have a percentage of clay content somewhere between 15 per cent and 25 per cent. I personally love clay-based soils. Providing they’re respected and never worked on when wet, they grow fantastic plants with much less resource input than sandy soils.

Loams
Loams are essentially mixtures of the first three. They are ideal for growing most plants. When you are aiming to create a loam, the quest is to determine what soil texture you have then add the correct proportions of other soil types to achieve the desirable blend.

Loams are the happy medium. They generally contain approximately 40 per cent by volume sand, 40 per cent by volume silt, and 20 per cent by volume clay. Loam is well drained, it takes up and holds moisture, it holds nutrients and has a stable overall structure that is relatively easy to work. Loam with a generous quantity of organic matter is the ideal garden soil.

The soil texture triangle should probably be familiar to you.

The best method of determining percentages of your particle sizes is to perform a jar test. Jump onto the internet for instructions if you are not familiar with the test.

Amending your soil texture with other soils such as clay or silt to sand is achievable on smaller scales but keep in mind that the addition of sand to clay can be problematic indeed. Unless you can provide over one third of coarse sand by volume to the clay, you are essentially making bricks. The tiny clay particles cling to the larger sand particles. The sand basically acts the same as gravel does in concrete. The best approach to amending clay soil is to allow it to dry, dress with 50 – 100mm of composted organic matter and break it open with an excavator, electric jackhammer with a clay spade or a hydraulic powered rotary hoe. Regardless of what approach you are embarking on, it is important to thoroughly blend the organic matter through the clay soil. If it is left in clumps it will interfere with water and air movement and root growth.

Amending a soil is not the same thing as mulching, although many mulches also are used as amendments. A mulch is left on the soil surface. Its purpose is to reduce evaporation and runoff, inhibit weed growth, and create an attractive appearance. Mulches also moderate soil temperature, helping to warm soils in the spring and cool them in the summer. Mulches may be incorporated into the soil as amendments after they have decomposed to the point that they no longer serve their purpose.

What soil amendment and how much should I use? I hear you ask.

Let’s cover them in future articles, in the meanwhile, I’d be calling “Murray” our soil scientist.

Merry Christmas to you all.

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